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AFRL-AFOSR-JP-TR-2018-0004

Simulating the mechanics of ultra-high performance fibre reinforced concrete for rapid low costmaterial development and analysis

Philip VisintinTHE UNIVERSITY OF ADELAIDE

Final Report12/18/2017

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4. TITLE AND SUBTITLESimulating the mechanics of ultra-high performance fibre reinforced concrete for rapid low cost material development and analysis

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5b. GRANT NUMBERFA2386-16-1-4098

5c. PROGRAM ELEMENT NUMBER61102F

6. AUTHOR(S)Philip Visintin

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7. PERFORMING ORGANIZATION NAME(S) AND ADDRESS(ES)THE UNIVERSITY OF ADELAIDE115 GRENFELL STADELAIDE, 5000 AU

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13. SUPPLEMENTARY NOTES

14. ABSTRACTThis research has: (i) identified the fundamental mechanisms influenced by the presence of fiber reinforcement; (ii) shown how each of these mechanisms can be modelled numerically; (iii) developed simple and cheap experimental procedures for extracting each of the material models required for analysis and; (iv) shown how the analysis techniques and material tests can be applied to predict the full range of behavior of UHPFRC flexural members. Significantly, this should reducing the need for undertaking large scale member tests thereby increasing the speed and reducing the cost of material development.

15. SUBJECT TERMSConcrete, UHPFRC, Ultra-high performance, fibre-reinforced, fiber-reinforced, Strength, Ductility

16. SECURITY CLASSIFICATION OF: 17. LIMITATION OFABSTRACT

SAR

18. NUMBEROFPAGES 21

19a. NAME OF RESPONSIBLE PERSONROBERTSON, SCOTT

19b. TELEPHONE NUMBER (Include area code)+81-042-511-7008

Standard Form 298 (Rev. 8/98)Prescribed by ANSI Std. Z39.18

Final Report for AOARD Grant 16IOA098 Simulating the mechanics of ultra-high performance fibre reinforced concrete for rapid

low cost material development and analysis 18 December 2017

Phillip Visintin: e-mail address: [email protected] Institution: The University of Adelaide

Postal address: School of Civil Environmental and Mining Engineering The University of Adelaide North Terrace, Adelaide, 5005

Phone: +618 8313 3710 Fax: +618 8303 4359

Period of Performance: 10/03/2016 – 10/03/2017

Abstract

The addition of randomly distributed fibres significantly alters the performance of reinforced concrete elements. Fibres that bridge flexural cracks provide a concrete tensile force in addition to that of conventional reinforcement. These enhance member flexural capacity, and reduce member deflection and crack width. Fibres that bridge sliding planes in the compression region allow for stable sliding during the formation of concrete softening wedges, leading to an increase in member ductility during overload scenarios. Together, these improvements in performance make fibre reinforced concrete an ideal construction material for structures of high importance.

A significant amount of testing has quantified the improvement in in concrete material properties (compressive and tensile stress strain behaviour) arising due to the addition of fibres. It is however difficult to incorporate these directly into existing strain based member analysis approaches as they cannot directly accommodate the rigid body deformations which occur as during crack formation and widening and wedge sliding. That is traditional approaches cannot model the behaviours most strongly influenced by fibres. This project seeks to address this shortcoming by quantifying the fundamental mechanics that govern the flexural behaviour of fibre reinforced concrete.

Having quantified the fundamental mechanics of fibre reinforced concrete, it is shown how these can be applied to reduce the cost and increase the speed of developing new types of fibre reinforced concrete. This is important as the almost infinite combination of fibre type and dosage rates make empirically based design approaches unfeasible.

The remainder of the report is presented through a series of published and submitted manuscripts as follows.

Chapter 1 contains the paper ‘Fundamental mechanics that govern the flexural behaviour of reinforced concrete beams with fibre-reinforced concrete’. This paper describes the influence of fibres on the fundamental mechanisms of crack formation, tension stiffening and concrete

DISTRIBUTION A. Approved for public release: distribution unlimited.

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to concrete sliding. It is shown how the influence of fibres on these fundamental mechanics can be incorporated into numerical analyses, an example is provided in terms of predicting the strength, ductility and crack width behaviour of normal strength concrete beams and columns with a small quantity of steel fibres. Chapter 2 contains the submitted manuscript titled: ‘Time dependent tension stiffening mechanics of fibre reinforced and ultra-high performance fibre reinforced concrete’. This work takes the tension stiffening mechanics formulated and solved numerically in chapter 1 and shows how it can be solved analytically with closed form solutions. This forms a significant first step towards developing design procedures for UHPFRC. Importantly, the solutions developed are generic enough to take any form of material tensile response, and unlike the current state-of-the-art do not require calibration from experimental testing. Chapter 3 contains the 3 submitted manuscripts and 1 departmental report: ‘Blending Macro and micro fibres to enhance the serviceability behaviour of UHPFRC’, ‘Shear friction behaviour of ultra-high performance fibre reinforced concrete’, ‘Local bond slip behaviour of steel reinforcing bars embedded in UHPFRC’ and the report ‘Compressive stress-strain behaviour of UHPFRC.’ These manuscripts describe suggested experimental procedures required to extract the fundamental material properties required for the generic analysis approaches developed in chapter 2 and 3. In each paper, after presenting an experimental procedure an example of application is carried out on UHPFRC with blended fibres. Chapter 4 rounds out the project by showing how each of the approaches and experimental procedures in chapters 1-3 can be applied to simulate the full load deflection response of a series of UHPFRC beams and slabs. Significantly, the manuscript ‘Mechanics of the flexural behaviour of UHPFFRC beams under instantaneous and sustained loading’ shows how only a small quantity of cheap, small scale tests are required to extract the relevant information required to predict the load deflection and load crack width behaviour of UHPFRC at all load levels. This research has: (i) identified the fundamental mechanisms influenced by the presence of fibre reinforcement; (ii) shown how each of these mechanisms can be modelled numerically; (iii) developed simple and cheap experimental procedures for extracting each of the material models required for analysis and; (iv) shown how the analysis techniques and material tests can be applied to predict the full range behaviour of UHPFRC flexural members. Significantly, this should reducing the need for undertaking large scale member tests thereby increasing the speed and reducing the cost of material development. Future research is suggested to further develop analytical solutions for mechanisms influenced by fibre reinforcement (similar to those developed in Chapter 2 for tension stiffening). This research could then be applied: (i) directly as a design approach for consulting engineers; (ii) as the basis of fast running numerical models, or; (iii) as the basis for desktop studies into how specific material behaviour influences member behaviour, without the need for empirical testing.


List of Publications and Significant Collaborations that resulted from your AOARD supported project:

a) Papers published in peer reviewed journals: • Visintin, P. and Oehlers, D.J., 2017. Fundamental mechanics that govern the

flexural behaviour of reinforced concrete beams with fibre-reinforced concrete. Advances in Structural Engineering, p.1369433217739705.

b) Manuscripts submitted but not yet published:

• Sturm, A.B., Visintin, P. Oehlers, D.J., Seracino, R. (2017).Time dependent tension stiffening mechanics of fibre reinforced and ultra-high performance fibre reinforced concrete. Submitted to ASCE Journal of Structural Engineering.

• Visintin, P., Sturm, A.B., Mohamed Ali, M.S., Oehlers, D.J. (2017). Blending Macro and micro fibres to enhance the serviceability behaviour of UHPFRC. Submitted to Cement and Concrete Composites

• Sturm, A.B., Visintin, P. Farries, K., Oehlers, D.J. (2017).Shear friction behaviour of ultra-high performance fibre reinforced concrete. Submitted to ASCE Journal of Materials in Civil Engineering.

• Sturm, A.B., Visintin, P. (2017). Local bond slip behaviour of steel reinforcing bars embedded in UHPFRC. Submitted to Structural Concrete.

• Sturm, A.B., Visintin, P., Oehlers, D.J. (2017). Mechanics of the flexural behaviour of UHPFFRC beams under instantaneous and sustained loading. Submitted to Engineering Structures.

c) Manuscripts published as departmental reports: • Sturm, A.B., Visintin P. (2017). Compressive stress-strain behaviour of

UHPFRC. Departmental Report, No. R198, School of Civil, Environmental & Mining Engineering, University of Adelaide, Adelaide, South Australia, Australia.

Attachments: Publications in categories a), b) and c) listed above.


Chapter 1

• Visintin, P. and Oehlers, D.J., 2017. Fundamental mechanics that govern the flexural behaviour of reinforced concrete beams with fibre-reinforced concrete. Advances in Structural Engineering, p.1369433217739705.


Chapter 2 • Sturm, A.B., Visintin, P. Oehlers, D.J., Seracino, R. (2017).Time

dependent tension stiffening mechanics of fibre reinforced and ultra-high performance fibre reinforced concrete. Submitted to ASCE Journal of Structural Engineering.


Chapter 3 • Visintin, P., Sturm, A.B., Mohamed Ali, M.S., Oehlers, D.J. (2017).

Blending Macro and micro fibres to enhance the serviceability behaviour of UHPFRC. Submitted to Cement and Concrete Composites

• Sturm, A.B., Visintin, P. Farries, K., Oehlers, D.J. (2017).Shear friction behaviour of ultra-high performance fibre reinforced concrete. Submitted to ASCE Journal of Materials in Civil Engineering.

• Sturm, A.B., Visintin, P. (2017). Local bond slip behaviour of steel reinforcing bars embedded in UHPFRC. Submitted to Structural Concrete.

• Sturm, A.B., Visintin P. (2017). Compressive stress-strain behaviour of UHPFRC. Departmental Report, No. R198, School of Civil, Environmental & Mining Engineering, University of Adelaide, Adelaide, South Australia, Australia.


Compressive stress-strain behaviour of UHPFRC

Sturm, A.B. and Visintin, P.

ABSTRACT

In this paper the compressive behaviour of UHPFRC manufactured from conventional materials and reinforced with a blend of short straight and long hooked steel fibres is experimentally characterised. To achieve this 18 tests were performed on 210 mm x 100 mm x 50 mm prisms to determine the stress-strain relationship and 43 tests were performed on cylinders to determine the compressive strength, elastic modulus and Poisson’s ratio. Shrinkage prisms were also monitored for 5 of the 6 mix designs. From this, the blending of fibres was found to effect the residual compressive stress-strain behaviour however other parameters were unaffected.

INTRODUCTION

UHPFRC is an advanced concrete technology first developed in Denmark in 1986 by Aalborg Portland (Buitelaar 2004). This material is characterised by the very low water to binder ratios (<0.2) as well as the removal of coarse aggregates and the introduction of steel fibres. The low water contents serve to cause a reduction in porosity resulting in a dense concrete matrix. The removal of coarse aggregates removes defects that can be a source of microcracks under compressive loads. Finally, the introduction of fibres results in strain hardening and ductile behaviour under tension. The introduction of fibres however complicates the development of mix designs due to the variety of different fibres that are available, particularly as previous research (Markovic 2006) has indicated that blends of fibres can have synergistic effects.

In this paper a number of key design material properties will be evaluated. These include the compressive stress-strain relationship, concrete strength, elastic modulus and the Poisson’s ratio. The last parameter is important as it relates the axial and lateral stress-strain relationships within the elastic regime. It also allows the determination of the shear modulus from the elastic modulus. The development of shrinkage was also monitored.

MIX DESIGN

The mix design is based on that proposed by Sobuz et al. (2017) and is shown in Table 1.

Table 1. Mix design for experimental programme

Mix designatio

n

Cement

(kg/m3

)

Sand (kg/m3

)

Silica fume

(kg/m3

)

Water (kg/m3

)

Superplasticiser (kg/m3)

Macro fibres (kg/m3

)

Micro fibres (kg/m3

) No fibres 978 973 260 171 44 0 0 1 Macro: 0 Micro

950 951 253 161 43 222 0

0.6 Macro:

0.4 Micro

950 945 253 166 43 88 133


0.5 Macro:

0.5 Micro

950 944 253 167 43 111 111

0.4 Macro:

0.6 Micro

950 944 253 167 43 133 88

0 Macro: 1 Micro

950 943 253 168 43 0 222

The macrofibres were 35 mm long and 0.55 mm diameter hooked end steel fibres with a yield strength of 1100 MPa and the microfibres were 13 mm long and 0.5 mm diameter straight steel fibres with a yield strength of 2850 MPa. The sulphate resisting cement had a fineness modulus of 365m2/kg, a 28 day compressive strength as determined in accordance with AS 2350.11-2006 (Standards Australia 2006a) of 60MPa and a 28 day mortar shrinkage strain determined in accordance with AS 2350.13-2006 (Standards Australia 2006b) of 650x10-6/mm was used along with an undensified silica fume that had a bulk density of 625 kg/m3. The sand was a washed river sand and had a fineness modulus of 2.34. A third generation high range water reducer with an added retarder was used to improve the workability

The mixing procedure consisted of first mixing all the dry components for 1 minute in the pan mixer until well combined. The water and superplasticiser were then added and the concrete mixed until visibly flowable. After the concrete started to flow, the fibres were added and mixed for a further 5 minutes.

TEST SETUPS

Compressive stress-strain relationship

The compressive stress-strain relationship was obtained from prisms with the dimensions highlighted in Fig. 1(a-b). The prisms were loaded at 0.07 mm/min until the peak load was reached, then 1 mm/min till a displacement of 5 mm was reached and finally at 2 mm/min until a displacement of 20 mm was reached. The deformation was measured by four LVDTs situated platen to platen. A failed specimen demonstrating a sliding wedge type failure is illustrated in Fig. 1(c).


Figure 1. Compression prism

Compressive Strength, Density, Elastic Modulus and Poisson’s Ratio

The density, compressive strength, elastic modulus and Poission’s ratio was estimated from cylinders with dimensions illustrated in Fig. 2(a-b). For each specimen the density was determined by weighing the specimens and measuring the dimensions. For specimens without strain gauges the compressive strength was simply obtained in accordance with AS1012.9-1997 (Standards Australia 1997a). This involved loading the specimen in uniaxial compression at a rate of 20 MPa/min until the peak load was reached. A failed specimen is indicated in Fig. 2(c). For the specimens with strain gauges the elastic modulus, Poisson’s ratio and compressive strength was obtained. First the elastic modulus and Poisson’s ratio were determined according to AS1012.17-1997 (Standards Australia 1997b). This involved loading and unloading the specimen up to 40% of the peak compressive load for five cycles of which the last two were recorded to determine the elastic modulus and Poisson’s ratio. A load rate of 15 MPa/mm was utilised at this stage of the test. Following this the cylinder was then loaded up to failure at 20 MPa/mm as was done for the specimens without strain gauges. Note that the specimens without strain gauges were tested first to determine the appropriate peak load for determining the elastic modulus and Poisson’s ratio. For the strain gauged specimens two vertical strain gauges were provided on each side to determine the elastic modulus and two horizontal strain gauges were provided on each side to determine the Poisson’s ratio.


Figure 2. Compression cylinder

Shrinkage

For five of the six mixes (excluding 1 Macro: 0 Micro) the total shrinkage was monitored on 75 mm x 75 mm x 275 mm concrete prisms in accordance with AS1012.8.4:2015 (Standards Australia 2015).

TEST RESULTS

Compressive stress-strain relationship

The average compressive stress-strain relationships for each mix are shown in Fig. 3 while the individual results are shown in Appendix A. From this it can be seen that for the mix without fibres due to the very high brittleness of this specimen failed immediately at the peak load without a post-peak response. For the specimens with fibres it can be seen that the stress-strain relationship is approximately tri-linear with an elastic ascending phase followed by a steep descending phase followed by a residual branch. 0 Macro: 1 Micro had the highest residual strength followed by 0.4 Macro: 0.6 Micro. The lowest residual strength was for 0.5 Macro: 0.5 Micro.


Figure 3. Average compressive-stress strain behaviour for each mix

Compressive Strength and Density

For 1 Macro: 0 Micro 5 specimens were tested at 27 days and 3 specimens were tested at 40 days with one specimen tested at 219 days. For all the other mixes for the first two testing times 3 specimens were tested and one specimen was tested at the final test time. The results are contained in Table 2 and plotted versus time in Fig. 4. From Fig. 4 it can be seen that there is negligible strength development after 20 days and that there appears to be negligible effect due to the presence of fibres as well. Considering all the results together the mean compressive strength is 158.6 MPa and the characteristic strength is 146.0 MPa.

The density was also measured for each specimen. It was noted that due to the higher density of steel (7850 kg/m3) the density of the concrete with 2% fibres was 8% higher than that without fibres. The average density of UHPC without fibres was 2328 kg/m3 and the upper characteristic value of the density is 2340 kg/m3. The average density of the UHPFRC was 2517 kg/m3 and the upper characteristic value of the density is 2549 kg/m3.


Table 2. Compressive strength and density

Mix Age at Testing (days)

fc (MPa)

ρ (kg/m3)

Age at Testing (days)

fc (MPa)

ρ (kg/m3)


fc (MPa)

ρ (kg/m3)

No Fibres 29 150 2330 54 169 2325 166 172

1 Macro: 0 Micro 27 160 2535 40 171 2542 219 167 2546

0.6 Macro: 0.4 Micro 27 157 2513 48 157 2513 99 162 2522

0.5 Macro: 0.5 Micro 63 151 2506 84 157 2522 142 150 2527

0.4 Macro: 0.6 Micro 44 156 2509 65 157 2511

0 Macro: 1 Micro 24 154 2500 48 156 2489 157 164 2517


Figure 4. Compressive strength versus proportion of microfibres and age


Elastic Modulus and Poisson’s Ratio

In Table 3 the elastic modulus and Poisson’s Ratio is recorded for each testing period. For 1 Macro: 0 Micro three specimens were tested at each testing period and for the first testing period for No Fibres and 0 Macro: 1 Micro. For the remaining testing periods only one specimen was testedThe average elastic modulus is 49900 MPa and the lower characteristic value is 46000 MPa. The average Poisson’s ratio is 0.226 and the characteristic value is 0.208. The values of the elastic modulus and Poisson’s ratio do not appear to be effected by age of testing or fibres.

Table 3. Density, Elastic modulus and Poisson’s ratio

Mix Age at Testing (days)

Ec (MPa) ν


Ec (MPa) ν

No Fibres 29 48158 0.231 54 47864 0.227

1 Macro: 0 Micro 27 51061 0.224 40 48453 0.227

0.6 Macro: 0.4 Micro 27 49073 0.217 48 52013 0.229

0.5 Macro: 0.5 Micro 63 49314 0.204 84 58202 0.259

0.4 Macro: 0.6 Micro 44 50060 0.230 65 51521 0.226

0 Macro: 1 Micro 24 50402 0.227 48 49538

0.229

Note: Ec is the elastic modulus and ν is the Poisson’s Ratio

Shrinkage

In Fig. 5 the smoothed shrinkage results are shown, with the individual results shown in Appendix B. From the plots it can be seen that the shrinkage strain develops quickly till about 40 days followed by a long plateau. There is considerable variation in the shrinkage strains developed but this appears to be a function of the time at which the concrete was cast and cured as opposed to fibre type. It should be seen that higher shrinkage values were obtained for the concrete cast during the summer while lower shrinkage values were obtained for concrete cast later during the autumn. This indicates that the environment effects the shrinkage behaviour of this material.


Figure 5. Variation of shrinkage with time

CONCLUSION

For a UHPFRC manufactured from conventional materials a series of key parameters for design have been characterised including: the axial stress strain relationship, compressive strength, elastic modulus, density, Poisson’s ratio and shrinkage. It was found that the fibre type has some effect on the residual compressive behaviour however the other parameters were unaffected. However, the quantity of shrinkage appears to be effected by the environmental effects.


APPENDIX A RAW STRESS-STRAIN RESULTS

Figure A1. Compressive stress-strain relationship


APPENDIX B RAW SHRINKAGE RESULTS

Figure B1: Variation of shrinkage with time


ACKNOWLDEGEMENTS

This material is based upon work supported by the Air Force Office of Scientific Research under award number FA2386-16-1-4098.

REFERENCES

Buitelaar, P. (2004). “Heavy reinforced ultra high performance concrete.” Proc., Int. Symp. on UHPC, Kassel, Germany,25-35.

Markovic, I. (2006). “High-performance hybrid-fibre concrete- development and utilisation”, Ph.D. Thesis, Delft University of Technology, Delft, Netherlands

Sobuz, H. R., Visintin, P., Ali, M. M., Singh, M., Griffith, M. C., and Sheikh, A. H. (2016). “Manufacturing ultra-high performance concrete utilising conventional materials and production methods.” Construction and Building Materials, 111, 251-261.

Standards Australia (1997). “Determination of the static chord modulus of elasticity and Poisson's ratio of concrete specimens.” AS1012.17-1997, Sydney, Australia

Standards Australia (2000). “Methods of testing concrete- Compressive strength tests - Concrete, mortar and grout specimens Standards Australia.” AS1012.9-2000, Sydney, Australia

Standards Australia (2006a), “Methods of testing portland, blended and masonry cements Determination of drying shrinkage of cement mortars.” AS2350.11-2006, Sydney, Australia

Standards Australia (2006b), “Determination of drying shrinkage of cement mortars.” AS2350.13-2006, Sydney, Australia

Standards Australia (2015), “Method for making and curing concrete-Drying shrinkage specimens prepared in the field or in the laboratory.” AS1012.8.4:2015, Sydney, Australia


Chapter 4 • Sturm, A.B., Visintin, P., Oehlers, D.J. (2017). Mechanics of the

flexural behaviour of UHPFFRC beams under instantaneous and sustained loading. Submitted to Engineering Structures.


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